I took a screenplay writing course this weekend. My intention is to turn my novel, THE BEAUTIFUL EVIL into a screenplay. This was a 2-day class for beginners.
The class fluctuated between study of the craft (noticed I wrote craft instead of art? The instructor made this very clear.) and portions of different films that represented the points she'd made.

This form, to the left, which I hope you can enlarge, is a great tool for screenwriters. In case it won't enlarge, I'll go over what this form actually does.
First, a screenplay is divided into 3 acts.
(The small lines in each category is the space to write the main points.
Act I: THE INTRODUCTION.
1) The yellow on the chart says: Opening scene where, guess what? you write points regarding the opening scene.) The main characters are introduced in Act 1.
2) Next there is the inciting incident (change to story that sets the character in motion.) This is denote in the red area.
3) The purple heading is for the B plot. This is the love relationship. This doesn't need to be two people. It could be a woman and her dog, a man (ie Tom Hanks) and his ball, (in Cast Away) etc..
4) Finally, the Turning Point (the lower red heading.) that moves the story in a different direction.
ACT II is broken down into two parts.
Part A: Fun & Games
1) These are points that continue moving the story forward with no major obstacles
2) The Dark Purple denotes the relationship from act 1 Building.
3) Next is the turning point that moves the story in a different direction.
Part B: Bad Guys Set In
1) Obstacles build, becoming increasingly more difficult.
2) Antagonist becomes more active
3) Relationship is in trouble (the purple on the sheet)
4) The red is the "All is lost moment:" the house burns down, someone dies, loses job, etc.
ACT III: Climax and Resolution
1) A decision is made.
2) Call to arms 2a The relationship survives. (Purple.)
3) Battle (Climax) In red on chart
4) Resolution
Try using this system and you'll see how easy it is to come up with a story.